1. Field of the Invention
The present invention relates to magnetrons used to generate high power electromagnetic radio frequency (RF) energy, and more particularly, to an apparatus disposed within a magnetron cathode for preventing undesirable physical contact between the heater filament of the cathode and the internal surface of the cathode.
2. Description of Related Art
Magnetrons have been used for many years in electronic systems that require high RF power in the microwave frequency range, such as radar systems. A magnetron typically includes a cylindrically shaped cathode that is coaxially disposed within an anode structure to define an interaction region between the cathode surface and the anode. The anode structure may include a network of vanes which provides a resonant cavity tuned to a frequency of oscillation for the magnetron. The microwave power produced by the magnetron is then coupled into an output waveguide that directs the power into a load, such as an antenna or other device. An example of a magnetron is found in U.S. Pat. No. 5,495,145 for PSEUDO-SPRING LOADING MECHANISM FOR MAGNETRON TUNER, which is incorporated herein by reference.
An internal heater is provided below the surface of the cathode, and is used to heat the cathode surface to a high temperature to produce thermionic emission of electrons therefrom. An electric potential applied between the cathode and the anode provides an electric field across the interaction region which causes the emitted electrons to form into a space-charge cloud. A magnetic field is provided along the cathode axis, perpendicular to the electric field, which causes individual electrons within the electron cloud to spiral into cycloidal paths in orbit around the cathode. When RF fields are present on the anode structure, the rotating electron cloud is concentrated into a spoke-like pattern. This is due to the acceleration and retardation of electrons in regions away from the spokes. The electron bunching induces high RF voltages on the anode structure, and the RF power level build up until the magnetron is drawing peak current for any given operating voltage. Electron current flows through the spokes from the cathode to the anode, producing a high power RF output signal at the desired frequency of oscillation.
In certain types of magnetrons, the internal cathode heater comprises a filament having one or more wire helixes connected in parallel that are rigidly connected at their respective ends. The wire helixes extend roughly the entire length of the cathode. An electrical current conducted through the wire helixes causes heat to be radiated onto the internal surface of the cathode to provide a uniform surface temperature. In some cases, the high filament temperature (e.g., 1,900.degree. C.) causes the wire helixes to expand and bow outwardly. This thermal expansion can cause the wire helixes to contact the internal surface of the cathode and produce an electrical short circuit between the filament and the cathode. The electrical shorting can cause the heater current to bypass a portion of the filament, causing uneven or incomplete heating of the cathode surface. As noted above, the cathode temperature is critical to thermionic electron emission, and improper heating of the cathode will therefore result in a reduction of the electron emission. Moreover, the electrical shorting can also cause excess heater current to be drawn from the magnetron power supply. These effects result in a reduced life expectancy of the magnetron.
Thus, it would be desirable to provide a magnetron cathode having a filament heater in which thermal expansion of the wire helix is controlled to prevent shorting of the wire helix to the internal surface of the cathode.